A FEW NOTES ON PLANT SIZES AND GROWTH RATES

    This is a difficult topic for any garden writer or catalog, for it is very clear that sizes are variable due to climatic conditions. The same plant might grow to an “ultimate or final size” of 3’ in New England, 4-5’ in Oregon, and 8’ in the deep south, if it will in fact grow successfully in all these areas of the country. Day length, plant nutrition & fertilizer rates, irrigation regimens & rainfall patterns, soil structure, elevation, wind direction and velocity, insect & disease setbacks, genetically regulated growth potential and cloud cover are just some of the many, many factors involved in determining rate of growth and ultimate size of an individual cultivar.

    For the garden conifers in particular, the size issue has become so confused that a classification system of sizes and growth rates has been developed by the American Conifer Society.

This is summarized as follows:

 

MINIATURE—less than 3” per year of growth;
      grows to less than 2-3’ in 10-15 years;
DWARF—3-6” per year annual growth;
      grows to 3-6' in 10-15 years;
INTERMEDIATE—6-12” per year annual growth;
      grows to 6-15’ in 10-15 years;
LARGE—12” or more per year annual growth;
      grows to more than 15’ in 10-15 years.

 

    Notice that the upper size limits are expressed in terms of 10-15 years of growth; ultimate sizes are not noted at all. While this system is helpful in terms of short-term residential gardens, it is of little help when dealing with older plants and arboretum-type situations. It is generally true that many cultivars do most of their growing in adolescence, and slow down as they approach maturity.

    The sizes that I might quote have been arrived at by personal observation in zones 6-7 in western North Carolina and are used strictly as a frame of reference; these are the growth rates and sizes I personally have been able to achieve, without pushing the plants very hard. I try to optimize conditions but do not use too much fertilizer. Your results will probably be different. Remember that in gardening, bigger and faster is rarely better—it just means that your display will become crowded more quickly.

    Many of our field plantings have taken 10 years or more to reach the sizes they display now. Thus, a customer can save considerable time by purchasing larger plants. While many gardeners enjoy “watching the plants grow,” others require that their gardens take substantial shape “now.” By making container and field grown plants available when we can, the customer has the option to inexpensively grow their own garden or to achieve instant gratification.

    The wise gardener will allow more room than usual between the principle woody plants in a garden display, and fill in with perennials, annuals, or smaller growing woodies. An alternative strategy in gardening is to plan to relocate certain plants after a definite period of time, such as on 5 year intervals. In this strategy, the major backbone plants do not move, and are spaced well apart from one another. Smaller growing woody plants can be initially grouped with a 3-4-5’ spacing, with plans to respace them later, perhaps expanding the entire garden in time. Other gardeners resort to the chainsaw to remove their problem-sized individuals. Perhaps a flexible combination of these techniques is best, but one should always keep an open mind on the subject. Not every plant will succeed, and a garden should always be changing to maximize viewer interest.